A connecting rod [1] shown in FIG. 1, which is a part of an automobile engine or the like, was made and connected to a complicated shaped crankshaft by the following process. A connecting rod body [2] and a connecting rod cap [3] were separately hot forged. They were subjected to thermal refining of “quenching and tempering”, boring of holes for bolts, finishing shaping, and finally assembling them by use of bolts [4] to connect the crankshaft.
However, due to recent severe economical conditions, the trend of reducing manufacturing costs in automobile's parts manufacturing reaches even in the region of the automobile's engine parts manufacturing. Thus, the demand for a non-heat treated connecting rod, which is not required in the thermal refining of “quenching and tempering” of the high manufacturing cost, has increased. Accordingly, in certain kinds of automobiles, a non-heat treated connecting rod has been adopted, which uses a steel which contains by mass %, 0.35% C-0.4% Si-0.95% Mn-0.04% S-0.5% Cr-0.1% V as basic chemical compositions. However, this non-heat treated connecting rod also needs the following conventional process. A connecting rod body [2] and a connecting rod cap [3] are separately hot forged. They are subjected to boring of holes for bolts, finishing shaping, and finally assembling them by use of bolts [4] to connect a crankshaft. Therefore the process of manufacturing of such a non-heat treated connecting rod are not satisfied to reduce the cost compared with a “fracture splitting connecting rod” which will be described later.
In recent years, in order to reduce manufacturing costs further, in addition to developing a non-heat treated steel for a connecting rod, the “fracture splitting connecting rod” manufactured by a following process are being considered. Both a connecting rod body part [2] and a connecting rod cap part [3] are formed integrally (in one body) by hot forging. It is “fracture split” (in other words, “fracture splitting”) at a Big end [5] into a connecting rod body [2] and a connecting rod cap [3].
For the above-mentioned “fracture splitting” treatment, a method of inserting a jig into the holes of the Big end [5] (e.g. N portions in FIG. 1) which are portions to be split of the integral forged materials, and loading a stress on the Big end to fracture by way of the jig, can be applied.
If the fracture surfaces are smooth and brittle in a fracture split connecting rod [1] (“a fracture splitting connecting rod”), the connecting rod body [2] and the connecting rod cap [3] can be assembled to connect a crankshaft properly, only by sandwiching a crankshaft between the connecting rod body [2] and the connecting rod cap [3] and connecting them with bolts in the state of putting their fracture surfaces together.
Therefore, if the fracture surfaces of the fracture splitting connecting rods are smooth and brittle, the machining process of the fracture surfaces of the connecting rod body and the connecting rod cap between which the crankshaft is sandwiched, is not needed, thereby reducing the manufacturing cost. Further, since the connection of the connecting rod body and the connecting rod cap are performed by fracture surfaces, an excellent rigid connection that also has excellent strength can be obtained.
With the above-mentioned fracture splitting connecting rod, a non-heat treated steel containing C (Carbon) of about 0.7% by mass as a material, disclosed in U.S. Pat. No. 5,135,587, has been actually used in Europe. However, this non-heat treated connecting rod used in Europe has inferior machinability compared to a conventional heat treated connecting rod with “carbon steels for machine structure use”, because of a high content of C. Thus, the non-heat treated connecting rod does not necessarily comply with a demand of the industry which prefers enhancing the machinability for bolt hole processing. Further, the non-heat treated connecting rod used in Europe is inferior in the fatigue limit (“fatigue limit” is hereinafter referred to as “fatigue strength” and is expressed by a mark of σw), compared to a conventional heat treated connecting rod with “carbon steels for machine structure use” and the aforementioned non-heat treated connecting rod, contains 0.35% C-0.4% Si-0.95% Mn-0.04% S-0.5% Cr-0.1% V by mass % as basic chemical compositions.
Therefore, there has been greatly increased a demand for a non-heat treated connecting rod having fracture splitting ability equal to or higher than that of actually used in Europe, and fatigue resistance equal to or higher than that of the said non-heat treated connecting rod with the basic chemical compositions of 0.35% C-0.4% Si-0.95% Mn-0.04% S-0.5% Cr-0.1% V by mass %, and excellent machinability. It is noted that the most typical method for increasing the machinability is to add Pb (lead) to the steel. However, from the viewpoint of protection of Earth's environment, a technique of increasing the machinability without the addition of Pb is required.
In Japanese Patent Laid-Open Publications Nos. 9-3589, 9-31594, 9-111412, 9-176785, 9-176786, 9-176787, 11-50184, 11-199967, 11-199968, 11-236643, 11-286746, 11-286750, 11-302778 and 2000-345298, a “low ductility non-heat treated steel”, in which the chemical compositions of the steel were controlled to enhance the fracture splitting ability, or a “low ductility non-heat treated steel”, in which the chemical compositions and carbosulfide of the steel were controlled to enhance the fracture splitting ability and machinability, have been disclosed. However, any non-heat treated steel proposed in these Japanese Patent Laid-Open publications fatigue resistance has not necessarily been considered enough.
Japanese Patent Laid-Open Publication No. 11-315340 discloses “steels for machine structural use excellent in fracture splitting ability and fatigue strength”, whose fracture splitting ability was enhanced, by making ductile ferrite brittle by reducing the Mn content and by increasing V content at a low carbon area of 0.2%-0.35% by weight, and by dispersing coarse TiC particles. However, the steels for machine structural use proposed in this Japanese Patent Laid-Open Publication contains only S (Sulfur) of 0.01% to 0.2% by weight for enhancing machinability. Thus the machinability required for the connecting rods cannot necessarily be satisfied.